In a gas chromatography-mass spectroscopy (GC-MS) instrument, gas chromatography is usually performed first and the resulting gas stream is then introduced into the mass spectrometer. As a result, both the carrier gas used during gas chromatography and the analytes become ionized and are directed as an ion beam into the mass spectrometer detector. While the ionization cross section for the carrier gas is typically more than ten times less than those of the analyte ions of interest, the carrier concentration is orders of magnitude greater than the analytes. As a result, the carrier ion concentration in the ion beam is many times more intense than the analyte ions.
One adverse consequence of this is that the electrostatic space charge effects due to the intense carrier ion concentration cause the ion beam to diverge. The divergence can be determined by the equation: ##EQU1## Where Z is the distance in which the beam diameter will double
r.sub.o is the initial radius of the beam PA1 I is the beam current PA1 V is the beam potential
The divergent beam may cause signal loss during detection if some of the beam falls outside the detector entrance. Another adverse consequence of the high carrier ion concentration is detector distortion or saturation. If the concentration causes the detector to exceed its linear range, its output will be distorted and the system may report erroneous results. In the event that the detector becomes saturated, those erroneous results will continue until the detector overcomes any inherent hysteresis.
One solution has been to gate the detector off during the arrival of the carrier ions, but this has the disadvantage of burdening the system with additional circuitry. Another solution has been to include an electrostatic deflection gate in the flight region that is activated during the passage of carrier ions, thereby preventing them from reaching the detector. This solution requires additional circuitry, additional mechanisms, precise timing and critical placement in the flight region.